The present invention relates to a speed change control apparatus and method for an automatic transmission in a vehicle. More particularly, it relates to a speed change control apparatus and method for downshifting.
In a conventional speed change control method for an automatic transmission in a vehicle, to carry out a downshift from the second speed to the first speed, for example, a controller performs feedback control separately on the solenoid valves. The solenoid valves supply oil pressure to the first-speed clutch and the second speed clutch, respectively, so as to increase the rotational speed of a transmission input shaft to the first-speed synchronous rotational speed. The changing rate of the input shaft speed is kept coincident with target values on the engagement- and disengagement-side clutches. This thereby engages the first-speed clutch with a specified timing while disengaging the second-speed clutch, so that the downshifting operation is completed.
According to this method, it is possible to implement a downshift irrespective of the driving condition (so-called power on or off state) of an engine.
The aforementioned conventional speed change control method, however, presents the following problem.
A case is considered in which the opening of a throttle valve suddenly increases during downshifting as shown by a curve "a" in FIG. 13. The sudden increase in the opening of the throttle valve leads to a sudden increase in the rotational speed of the transmission input shaft, as shown in FIG. 13 by the two-dot chain line curve "b" indicative of the time-dependent change of a turbine rotational speed Nt. In this case, the controller tries to suppress the sudden increase in the rotational speed of the input shaft by increasing the duty ratio of the second-speed solenoid valve, so that the oil pressure of the second-speed clutch can increase to match the changing rate of the input shaft rotational speed to the changing rate of the disengagement-side target value.
The hydraulic circuit system located between the second-speed clutch and the solenoid valve, however, has components such as an accumulator and orifice because of its structural necessity. Therefore, even when the duty ratio of the solenoid valve is increased, the oil pressure of the second-speed clutch does not immediately increase. This causes the controller to misjudge that the oil pressure of the second-speed clutch is still too low and therefore to further increase the duty ratio of the solenoid valve. As a result, the controller increases the working oil pressure of the second-speed clutch higher than necessary (This is shown in FIG. 13 by a two-dot chain line curve d3 indicative of the changes in the duty ratio of the solenoid valve with the elapse of time).
Thus, the oil pressure supplied to the second-speed clutch becomes higher than necessary, and hence the disengagement of the clutch is delayed. This results in "interlock" in relation to the first-speed clutch which is gradually being engaged. As a result, the torque of the output shaft significantly changes as shown in FIG. 13 by the two-dot chain line curve "c" indicative of the changes in the T/M output shaft torque with the elapse of time, presenting a problem of the occurrence of a significant speed change shock.